Trinitron 100HD Pool Filter Leak

I have a Trinitron 100HD Pool filter for my in ground pool. The filter is leaking outside around the filter bulkhead at the return pipe. It looks like an o-ring or gasket has blown out. Does this have to be repaird from inside the filter or can I replace the gasket or o-ring from outside the filter bulkhead? I can't tell from looking at the drawings of the filter and it's parts.

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